Another Jetting question

Dear nortonist fellows,

I've been unable to find in this forum the "supposed" correct jetting for an 850 fitted with KN airfilter (and fullauto head :roll: ) . I'm using Amal carbs with 220 jetting, which is the standard for a MkIIa, and I was wondering if that was not to lean. Plugs seem okay perhaps a bit lean, but I've got 260 jets on my shelves from the old carbs, are these worth a try ?

Thx

Xavier

Not exactly uncharted waters but not enough info out there to get what you need.

I really think you need to do this testing yourself and report back as to the best scenario for jetting the Fullauto head and the K&N filter. I doubt the main will change much from stock, maybe a size or 2, but who know what direction. A greater demand may bring the need to a larger jet but the efficiency of the unit may ask for a smaller one.

No two motors run exactly like the other. Even if someone comes up with a number, it should be treated as a baseline for a good starting point.

Sorry for the null answer.

A bit more info needed:

What type/octane of fuel are you using in France?

What type and heat range of spark plugs and ignition are on your bike?

I have 220 main jets on my 750 with 32mm intake ports and Amal carbs (3-1/2 slides) with the old model K&N filter, no problem,
but I rarely get onto the main jet metering anymore!

Anytime you REDUCE the upstream air restriction, you reduce the vacuum signal at the needle jet, and a larger jet(s) is needed to compensate. BUT, two SMALL K&N (rockscreens, IMHO) may flow LESS than one GIANT pleated paper element did. Plug chop color isn't what it once was, modern fuel doesn't color a plug until HOG RICH.

By asking this, I knew I would have almost as many answers as there are people on this forum But I'm trying to get a general idea about jetting. As you said, concours, modern fuel doesn't help with plug color.

I would think that K&N Filters are less restrictives and thus jetting should be enlarged to compensate for the bigger air quantity admitted.

More details :

- In France we usually run on Unleaded with 98 Octane,
- I have this type of KN :
- Plugs are BP7ES
- Boyer Ignition
- Standard amal carbs (new),


Moreover, I've noticed very recently that my engines dies when blipping too fast from idle, that's what make me think I might be running too lean... Anyway on the road, It's doing really reaaly fine. :wink:

Xavier

These highly common filters "can" be considered a lateral move. Your issue is more than likely pilot circuit or air screw and possibly the needle height.

Xpongebob said:

Moreover, I've noticed very recently that my engines dies when blipping too fast from idle, that's what make me think I might be running too lean... Anyway on the road, It's doing really reaaly fine. :wink:

Xavier

Click to expand...

Blipping from idle probably has nothing to do you your main jet. You might want to read this site's advice. Take the carbs apart and clean them. They can come with all kinds of stuff in them new. Different throttle settings correspond to the different circuits in the carb, pilot, slide, needle/neddle jet, and main. You need to take them on separately.

http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

Hi Bob,

You're right, that's one of the first things I've done : cleaning pilot circuit, checking fuel level. Seems to be okay.

So I decided to lift the needle, which was set on the middle notch, first notch. I Cleaned the coil contacts, and am thinking changing the plugs. Today it's running fine again, so let's wait and see.

What made me wondering is that for the 850 Mk. II you will find 3 different jet settings :

Mk.ll 1973 - 1974
Specification No. 1

Main jet 260
Slide 3½
Needle jet 106
Needle position 1 or 3


Mk.ll 1973 - 1974
Mk.lll 1975 - 1977
Specification No. 2

Main jet 230
Slide 3½
Needle jet 106
Needle position 2 or 3


Mk.lla 1974

Main jet 220
Slide 3½
Needle jet 106
Needle position 2


Why is that ?

Regards

Xavier

Bonjour Jan,

I've just ordered #3 Slides, several pairs of main jets, and make my trials.
Spray tubes are standard, I think, so straight cut, if I'm not wrong.

Si tu passes aux environs de Bordeaux, passes donc boire un coup, et tu me diras ce que tu en penses ! :wink:

A+

Xavier

Ah, interresting, I've thrown away the chokes without plugging the holes ! I'll do that, too.
I'll wait a little before cutting anything.

A la rigueur, je peux passer te voir avec la bête à l'occasion, si tu préfères et que le temps s'y prête, tu me diras.
Merci pour les conseils !

A+

Xavier

Xpongebob said:

What made me wondering is that for the 850 Mk. II you will find 3 different jet settings :

Mk.ll 1973 - 1974
Specification No. 1

Main jet 260
Slide 3½
Needle jet 106
Needle position 1 or 3


Mk.ll 1973 - 1974
Mk.lll 1975 - 1977
Specification No. 2

Main jet 230
Slide 3½
Needle jet 106
Needle position 2 or 3


Mk.lla 1974

Main jet 220
Slide 3½
Needle jet 106
Needle position 2


Why is that ?

Regards

Xavier

Click to expand...

I think one possible explanation, especially for different main jet sizes, could be that the differing published spec's are from different points in time and different locations, it may have to do with the gas and ethanol. Also the things these same writers did to their bikes, like aircleaners and float heights, especially float height. I couldn't get my bike to run well at low throttle settings with standard jetting and float height. I kept going to smaller pilot jetting but it was still way rich. Then I put the standard pilot jet and air screw setting back and dropped the float height well past the standard setting. The soot was gone and it ran like I put one of the spark plug caps back on. In car lingo, it's "tip in". Somebody called it, "The Empirical Method of Setting Float Height", just going with what your eyes and ears tell you.

Xpongebob said:

Ah, interresting, I've thrown away the chokes without plugging the holes ! I'll do that, too.
I'll wait a little before cutting anything.
Xavier

Click to expand...

I just re-read this part. Could be the problem is solved.

I suggest you need to recognize the functions of the different metering offered within the carburettor.

The mains operate at 3/4 to full throttle and are adjusted by plug reading.
The needles and needle jets operate from 1/4 to 3/4 throttle and affect the power used in tight twisty situations.
The pilot and slide cutaway operate at low throttle openings.

Lower the needles until motor coughs when the bike is used in tight twisty situations, then lift the needles one notch.
Look for the black band in the plugs when setting the mains. it should be evident on the porcelain right down inside where it meets the metal. If it is not there you could start burning pistons and valves.

I usually start by fitting larger mains, then adjust the needles so the bike gives good response and power around t he twisty stuff, then do the high speed blast and reset the mains. Lean is good, but your bike will go fastest just before it self-destructs. It pays to watch the weather and do your tuning when it is very cold, as the weather warms up the mixture will richen - the reverse can be a problem.

Set the float levels to the manufacturer's specification before changing anything to do with metering.

rpatton said:

Xpongebob said:

What made me wondering is that for the 850 Mk. II you will find 3 different jet settings :

Mk.ll 1973 - 1974
Specification No. 1

Main jet 260
Slide 3½
Needle jet 106
Needle position 1 or 3


Mk.ll 1973 - 1974
Mk.lll 1975 - 1977
Specification No. 2

Main jet 230
Slide 3½
Needle jet 106
Needle position 2 or 3


Mk.lla 1974

Main jet 220
Slide 3½
Needle jet 106
Needle position 2


Why is that ?

Regards

Xavier

Click to expand...

I think one possible explanation, especially for different main jet sizes, could be that the differing published spec's are from different points in time and different locations, it may have to do with the gas and ethanol.

Click to expand...

Only, they are original Norton/Amal factory specifications. Unfortunately they appear to have been copied incorrectly, I believe the list below to be more accurate.


Mk.1 & Mk.2 1973 - 74
Amal 932/R29 & L30
Main jet 260
Slide 3½
Needle jet 106
Needle position 1

Mk.1A 1973 -74
Amal 932/R31 & L32
Main jet 230
Slide 3½
Needle jet 106
Needle position ?

Mk.2A 1974 & Mk3 1975-77
Amal 932/R33 & L34
Main jet 220
Slide 3½
Needle jet 106
Needle position 1

850 carbs had 928/107 stepped spray tubes with 928/104 needles

ludwig said:

Bonjour Xavier ,
The size of the main jet is not all that important .
With less restrictive air intake an exhaust , something between 240 -260 will do .
( I use 230 )
IMO , more important is the cutoff of the air slide .
Instead of a # 3 1/2 , try a # 3 slide .
It will make the mixture richer in the mid range , and likely do away with the flat spot .
Throw out the choke slide ( you don't need that in Bordeaux ) but plug up the hole :

Leave the needle in the lowest position .
Float level is also important .
What type of spray tube do you have : stepped , or straight cut ?

Click to expand...

I've got a similar problem with my 750 cutting out after a blip, I had found the float height low but fixed that, the point you made about the choke slide getting plugged has me wondering though. I've taken my choke slide out completely and blanked off the cable adjuster but just left the main slide as is, does your picture show the choke slide fixed in place and the cable hole plugged? I'm running a single 32mm carb with a 260 main, 106 needle jet with the needle on the top notch in a 3.5 slide but have just received a 107 jet to try. Pilot route is clean and the tickover is even around 900-1000 when warmed up, anything else to look at carb side?

Apologies for the hi-jack but thats interesting. I've never seen the choke slide hole plugged or even recommended. Mine is chokeless and has been for 90% of the time I've owned it. It has never run well when hot, from idle through to where the cutaway comes in and having just put the head back and got the carbs bang on for equal lift its really noticeable that as the slides just move (a tiny fraction only) the engine stumbles and stalls. If I open the throttle a bit quicker I can bypass this characteristic. The carb's cable choke hole is blocked so I guess its worth trying with the slide hole blocked but I'm not sure what that achieves since in normal use the choke slide is in the upper body of the carb. Just where would the (mixture weakening) air be coming from? Of course when its cold it runs beautifully so it all points to richness, I know :? but it doesn't seem that clearcut, well not to me.
I'm watching the slides with no aircleaner so it's a little weaker but I'm not sure what's going on? Opening the throttle too quickly it spits and dies (presumably too weak) or is it dying because its rich at some point? Std 106 needle jet and three groove needle in top groove, No. 3 slide. This is not a new issue, its always been poor despite new carbs, new bodies and slides etc. over the years.

Only, they are original Norton/Amal factory specifications. Unfortunately they appear to have been copied incorrectly, I believe the list below to be more accurate.

Click to expand...

Hi L.A.B. , I got these from the NOC website, there are not so far from yours apart from the needle position, are they ?

@acotrel :
Thanks for sharing ! I'm aware of the different ranges of carb operations, but I think that during transitional phases (quick opening) these tend to mix, don't they ?
I'm still breaking in the engine, so I'm not really able to get full throttle opening (I'm working around 4000 rpm), and the engine is doing really good, but I don't want my bike to self-destruct :wink: that's why I'm checking usual settings/customs for amal's, as I didn't choose the carb configuration first place, and I supect too lean a mixture.
And thx for the plug cheking tip ! :wink:

@willy mac :
I'm wondering myself how to correctly plug these !

oops messed up and couldn't delete...

Since Amals are have neither accelerator pumps nor constant velocity slides, blipping the throttle quickly from idle is always going to cause a stumble unless the idle/low speed setting is insanely rich or idle speed is extremely high.